While the chemical is harmless when used as a nutritional supplement or in skin care products, when inhaled it may pose a health risk, the health department warned. The department said it had received reports of 34 people aged between 15 and 46 experiencing severe lung illness in New York who had used a cannabis-containing vape product that contained the vitamin E acetate.

"Vitamin E acetate is now a key focus of the department's investigation of potential causes of vaping-associated pulmonary illnesses," the department said in a statement.

Battery-operated e-cigarettes hit the US market about a decade ago, touted as a safer alternative to traditional tobacco cigarettes. But in recent months, an increasing number of people who vape are winding up in hospital with symptoms that include coughing, shortness of breath and fatigue that worsened over time.

The Centers for Disease Control and Prevention opened a probe into vaping-related lung disease in August. As of Aug. 27, the CDC said it had identified at least 215 possible cases in 25 states of severe lung disease that could be caused by vaping.

The Federal Drug Administration said Thursday it's working with the CDC in an investigation of the incidents. It's testing more than 100 samples as part of its investigation, analyzing them for the presence of a broad range of chemicals, including nicotine, THC, pesticides, opioids, poisons and toxins. But the agency added that it needed more information to determine whether there's a connection between the reported illnesses and any specific product or substance.

"No one substance, including Vitamin E acetate, has been identified in all of the samples tested," the agency said in a statement. "Importantly, identifying any compounds that are present in the samples will be one piece of the puzzle but will not necessarily answer questions about causality."